. Adsorption and Absorption Solid surfaces have the ability to attract the contacting species due to free valency or residual force on them. For example: charcoal adsorbs ammonia, silica gel adsorbs water., charcoal adsorbs colorants from sugar. These examples prove that adsorption is a surface phenomenon.
In contrast to adsorption, absorption is a bulk phenomenon i.e. the adsorbate molecules are distributed throughout the adsorbent. Adsorbent is the material on which adsorption takes place. Adsorbed substance is called an adsorbate.
The surface of separation of the two phases where the concentration of adsorbed molecule is high is known as interface. In adsorption, if the concentration of a substance in the interface is high, then it is called positive adsorption. If it is less, then it is called negative adsorption. The process of removing an adsorbed substance from the surface is called desorption.
The gaseous molecules like He, Ne,O ,N ,SO and NH and solutions of NaCl or KCl can be adsorbed by suitable adsorbents. These are referred as adsorbates. Silica gel and metals like Ni,Cu, Pt, Ag and Pd and certain colloids can act as adsorbents. XII U10-Surface XII U10-Surface - - - - Characteristics of adsorption .
Adsorption can occur in all interfacial surfaces i.e. the adsorption can occur in between gas-solid, liquid-solid, liquid-liquid, solid- solid and gas-liquid. . Adsorption is a spontaneous process and it is always accompanied by decrease in free energy.
When D G reaches zero, the equilibrium is attained. We know, D D D G = H - T S where D G is Change in Free energy. D H is Change in enthalpy and D S = Change in entropy. .
When molecules are adsorbed, there is always a decrease in randomness of the molecules. ie., ∆S< , and T∆S is negative. Hence, adsorption is exothermic. Adsorption is a quick process whereas absorption is a slow process.
M.C. Bain introduced a term ‘sorption’ to represent the simultaneous adsorption and absorption. T. Graham used a term occlusion for sorption of gases on metal surfaces.
. . Types of adsorption Adsorption is classified